That can get long and tiring if you aren't flying!
There are direct flights and trains for this journey. Catching a direct flight is the fastest way to do this journey. Direct trains are available as well. You can also travel there by taxi. New Delhi (Delhi) to Mumbai (Maharashtra) distance is 1428 km and you can get there by flight in just over 1 hour and 45 minutes. Going by train won't be that fast but you will get fares starting from ₹ 471.
The weather is amazing right now. You'll have a great time.